1. Field of the Invention
This invention relates to interferometric multimode fiber optic switches and modulators and, in particular, relates to electro-optical, multimode fiber optical switching or modulating devices, depending upon their modes of operation. Accordingly, it is a general object of this invention to provide new and improved devices of such character.
2. Description of the Prior Art
Switching and modulating devices of the prior art operate in various manners. Commonly, such functions are performed electronically and the final results are converted into optical information. All-optical switching apparatus are available in two general types: one type is a mechanical switch in which an input fiber is physically displaced between two stable configurations corresponding to alignment with one of two output fibers. Such motion is realized by either an electromagnetic activator or a piezoelectric transducer. A second type utilizes a thicklayer, multimode planar wave guide in an electro-optical substrate. Application of voltage to such substrate creates a discontinuity in index of refraction and, therefore, a change in the direction of propagation of the light beam, usually using total internal reflection for such purpose.
Modulation can be achieved in the prior art electronically by varying the current through the light source, per se, such as an LED (light emitting diode) or a laser diode.
Disadvantageously, electronic schemes of the prior art of switching and modulating introduce power conversion losses, additional noise, and nonlinearities into the characteristics of available light sources.
Disadvantageously, mechanical schemes of the prior art include numerous deficiencies, such as their inherent sluggishness due to inertia of moving parts. They usually require relatively high power and voltage due to their large and cumbersome sizes. They are subject to wear of moving parts, and, therefore, their reliability is not satisfactory.
Disadvantageously, the multimode planar wave guide techniques require mode conversion from the fiber cylindrical structure to the planar geometry, such conversion introducing significant losses. Furthermore, the many modes excited in the planar wave guide differ in propagation constants, and, therefore, the degree to which the electro-optical effect affects them varies according to mode parameter. For such reason, the "extinction ratio" (i.e., the ratio of the maximum amount of light out from the modulator to the minimum amount of light out from the modulator) is not very high, and reported results have not exceeded 20 dB.